This invention relates to the detection of short digital messages transmitted by radio, either terrestrial or relayed by satellite. More particularly the invention is directed to a novel method for effective synchronization and detection of short digital radio messages reliably under very noisy channel conditions.
Traditional digital radio transmission techniques use a single channel per carrier where one channel is dedicated to each user, the user transmissions are typically long in duration. Synchronization techniques for such systems often rely on long term averaging in order to work reliably. With greater demand for spectral resources, time division multiple access (TDMA), where multiple users share the same channel in a time ordered fashion, is becoming common. A current example is TDMA cellular telephony standard, see Ref 7, IS-54, TIA Interim Standard. With TDMA systems, the individual messages or bursts transmitted are often very short, so that very efficient and non-traditional synchronization techniques must be employed.
For short message transmission each burst typically includes a unique word, that is, a sequence of known bits or symbols, distributed in some manner throughout unknown data symbols making up the rest of the burst. The purpose of the unique word is to assist synchronization to the burst, in frequency, time, and phase. Synchronization in many current systems is also assisted through precompensation of the burst, so that uncertainty in time and frequency is limited to a small range. This precompensation information is obtained from feedback from the synchronization of previously transmitted bursts. This reduces the search range of the receiver synchronization circuitry, but does not preclude the necessity to perform fine synchronization for proper extraction of the data from the noise. It is with fine synchronization that the present invention is concerned.
Although forward error correction is employed to reduce the error rate, as lower power transmitters are deployed and radio channel environments become noisier, the raw channel bits become even less reliable before the forward error correction decoding is undertaken, and synchronization of the unique word becomes more crucial for synchronization. Furthermore for short bursts, the unique word length must be minimized to reduce the overhead (portion of the signal not carrying the data). A further constraint in mobile radios is that limited processing power and time is available. Thus, although greater demands are being placed on the synchronization techniques they still must be simple and practical enough to be implemented in a mobile terminal.
It is an object of this disclosure to provide a method, which is of relatively low complexity, for reliably synchronizing and detecting very short digital radio messages under very noisy channel conditions.
is a further object to provide a method for integrating synchronization, detection, and forward error correction decoding in such messages.
Here described is a multi-stage method for reliably detecting short digital messages. It assumes the message contains unique words, known at the receiver, and unknown data. The unique words are assumed to be multiple phase shift keying (MPSK) modulated, the preferred embodiment is binary phase shift keying (BPSK). The data portion of the burst may be MPSK modulated or multiple quadrature amplitude modulation (MQAM). The method described comprises a series of steps that produce successive refinements of the synchronization and detection process.
The method is implemented using a digital software receiver. That is, in the receiver, the received, modulated, RF signal is down-converted to an approximate complex baseband signal and then both in-phase and quadrature components are sampled by an analog to digital converter (A/D). The frequency uncertainty (the error in the down-conversion process) can be typically up to 10% of the symbol rate, beyond this the synchronization reliability decreases. The timing uncertainty can be any number of symbol periods but the synchronization reliability improves as the timing uncertainty decreases.
In the described method for more reliably detecting and decoding short digital messages received over a noisy channel, nine steps are preferred. The first step is to obtain initial frame synchronization for the received burst. The second to obtain an initial estimate of the carrier frequency error. The third is to correct this frequency error in the received samples. The fourth step is to obtain a refined timing estimate. The fifth step is to perform detection filtering, simultaneously correcting for the residual timing error and decimating to one sample per symbol. The sixth step is to estimate the phase and amplitude of the received burst and correct it. The seventh step is to obtain a refined frequency estimate and correct for it. The eighth step is to compute reliability estimates or, optionally, to make hard decisions for the individual bits defining each transmitted symbol. A ninth and optional step is to use the reliability estimates in a soft-input decoding algorithm.
In the process here described some of the steps are known in the prior art.
The first step of coarse timing (frame synchronization) is prior art, for example see, Ref. 1), R. A. Scholtz, xe2x80x9cFrame Synchronization Techniques,xe2x80x9d IEEE Trans. Commun., vol. COM-28, No. 8, Aug. 1980, pp. 1204-1213, which is included herein by reference. This step briefly comprises; differentially detecting the received signal over that time interval which potentially corresponds to the unique word (including the estimated uncertainty in this); correlating the result with the known differential unique word; and choosing the point of maximum correlation in the uncertainty window as the frame synchronization point.
The second step of coarse carrier frequency synchronization is also known in the prior art, for example, see Ref. 2), S. Crozier, xe2x80x9cTheoretical and simulated performance for a novel frequency estimation technique,xe2x80x9d Third Int. Mobile Satellite Conf., Jun. 16-18th, 1993, Pasadena, Calif., pp.423-428, which is included herein by reference. The steps of this algorithm are, briefly: using the soft symbol estimates implied by the timing estimate of the first step, remove the modulation from the signal (such as by multiplying by the conjugate, if using multiple phase shift keying MPSK). With the derived pure carrier modulation-removed signal, compute the average phase-differential between successive symbols of the unique word. In the third step, improve this phase differential estimate by correcting the derived carrier frequency by the initial phase-differential estimate. The frequency estimate and correction provided by the second and third steps can be further improved by estimating the phase-differential over more than one symbol period. Crozier discusses details on determining the best delay spacing.
The fourth step of fine timing estimation also draws partly upon the prior art, for example, see Ref. 3), A. D. Whalen, Detection of Signals in Noise, San Diego: Academic Press, 1971 and also see Ref. 4), H. L. van Trees, Detection, Estimation and Modulation Theory, New York: John Wiley and Sons, 1968, both of which are included herein by reference. These authors indicate that the maximum likelihood approach to obtaining the timing of a known signal is to correlate the noisy signal received with the known signal over the window of timing uncertainty. The time of peak correlation between the two corresponds to the optimum timing.
In this present disclosure, the known signal is the filtered unique word that is part of the transmitted burst, and correlation is performed in the discrete sample domain. The steps of this algorithm comprise:
i) perform a correlation at the timing given by the initial estimate of frame sync obtained from the first step above, and at one sample on either side of this;
ii) perform an interpolation between the magnitudes of the resulting correlations; and
iii) determine the time shift in terms of the offset (delay or advance) with respect to the coarse timing at which the interpolation peak occurs over the range of these three samples.
The preferred approach is to use a parabolic interpolation function. The timing error can then be corrected using a digital filter with a compensating timing offset. The preferred approach for the filter is to precompute a number of filters with relative fraction sample delays, e.g, 0, xc2xc,xc2xd,xc2xe (when using four fraction sample offsets), and select the one that most closely compensates the timing error.
Background to the sixth step of phase and amplitude estimation is described, for example, in Ref. 5), D. C. Rife and R. R. Boorsty, xe2x80x9cSingle-tone parameter estimation from discrete-time observations,xe2x80x9d IEEE Trans. Inform. Theory, Vol. IT-20, No. 5, September 1974., which is included herein by reference. In this algorithm, once one has an estimate of the timing of the unique word, one removes the unique word modulation. The result is a single-tone to which the prior art can be applied directly. As is well known in the art, if the unique word is MPSK modulated, the modulation can be removed by multiplying by the complex conjugate of known symbols.